Non-oxide ceramic products such as silicon carbide, silicon nitride, silicon boride, and/or boron nitride possess many desirable properties, such as a high thermal stability, a high oxidative stability, a high degree of hardness, and a wide range of electronic properties.
These ceramic products can be prepared by the pyrolysis of a suitable polymer precursor, according to methods described, for example, in Ann. Rev. Mater. Sci., 14:297 (1984); Polymer News, 16:6 (1991); J. Am. Ceram. Soc., 71:1104 (1988) and Inorganic and Organomet., ACS Symposium, Series 360, Washington, D.C. (1988). Silicon carbide ceramic fibers and various modifications thereof produced by the polymer-pyrolysis route have been disclosed in U.S. Pat. Nos. 4,052,430 and 4,604,367. These ceramic fibers, known as Nicalon silicon carbide materials, however, usually contain 11-13% of oxygen which is introduced during the curing process, causing the Nicalon silicon carbide ceramic fibers to have the lower tensile properties at or above 1100.degree. C. The low tensile properties of these ceramic fibers are presumably due to the presence of silicon oxides in the fibers, and the silicon-carbon grain size grown at high temperatures.
U.S. Pat. No. 4,604,367 discloses an inorganic fiber composed of silicon, carbon, boron, and nitrogen comprising a four-step process for the production of organoborosilicon precursor fibers. (A) In the first step, the constituent components are heated to produce 2 meltable polymer. (B) In the second step, the compound is spun into a precursor fiber. (C) In the third step, the precursor fiber is cured in an oxidizing atmosphere. The curing (or further) processing can also be achieved by gamma ray and/or e-beam irradiation in a vacuum or in an inert gas and by oxidation. (D) In the fourth step, the cured precursor fiber is calcinated by heating in the range of 900.degree. C. to 1800.degree. C. The resulting ceramic fiber is more stable at high temperatures.
U.S. Pat. No. 4,550,151 discloses another organoborosilicon nitrogen-containing fiber and method or its production. U.S. Pat. No. 4,942,011 relates to a silicon carbide fiber having a higher density. The patent also discloses a means for production of a ceramic fiber using a sintering process in an inert or reducing atmosphere at the temperature range of 1800.degree. C. to 2200.degree. C. in the presence of 0.2-5% of boron carbide as sintering aid.
U.S. Pat. No. 4,987,201 describes the preparation of an organo-metallic polymer comprising boron-carbon-silicon produced by pyrolysis of an inorganic Si-B-C material. Similar inorganic silicon-boron-carbon material is also discussed by M.-T. S. Hsu et al. in the Journal of Applied Polymer Science, Vol. 42, pp. 851-861, published in 1991. U.S. Pat. Nos. 4,767,728 and 4,851,491 also relate to the production of organoborosilicon polymers. Polymers of the silicon-boron carbon systems described in the 4,851,491 and 4,767,728 patents can be used as new ceramic precursors of the present invention.
Silicon nitride and polymeric precursors were reported in Am. Ceram. Soc. Bull., 66:363 (1987) to yield high tensile strength fibers upon pyrolysis. Further, the pyrolysis of silicon carbide polymers can produce ceramic fibers with engineering scale properties.
All references and patents cited herein are incorporated by reference in their entirety. However, none teach or suggest the present invention.
It is often desirable to obtain a ceramic in a particular final shape or form, e.g. a fiber. Often the precursor organic fiber is formed and then pyrolyzed directly to form the ceramic fiber. When the precursor polymer is pyrolyzed directly, the organic fiber melts and deforms during pyrolysis. Thus, the final ceramic fiber shape does not have the desired shape.
The current invention obviates many of the disadvantages connected with above cited patents. Namely, it provides a process to produce high strength silicon-boron-carbon ceramic fibers which are more stable at high temperatures than conventional silicon carbide fibers and silicon nitride fibers. The novel process also includes a curing (or crosslinking) step B of the precursor polymer so that it does not melt or deform during the subsequent pyrolysis.